The present invention relates to selectively hydrogenated block copolymers where at least one of the blocks is a random copolymer of a mono-alkenyl arene monomer, such as styrene, and a 1,3-cyclodiene monomer, such as 1,3-cyclohexadiene, and at least one of the other blocks is a hydrogenated polymer or copolymer of an acyclic conjugated diene, such as 1,3-butadiene or isoprene.
The preparation of styrene-diene block copolymers is well known. One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. These polymers in turn could be hydrogenated to form more stable block copolymers, such as those described in U.S. Pat. Nos. 3,595,942 and Re. 27,145. Over the years such styrene based thermoplastic elastomers have been used in a large number of applications and end-uses. However, for some of these end uses it is necessary that the polymer have good mechanical properties at elevated application or service temperatures, and the existing styrene-diene block copolymers have some deficiencies in that regard. One approach to improve the upper service temperature of such polymers is to increase the glass transition temperature (Tg) of the end-blocks of the polymer. An example of that approach has been the hydrogenation of the poly (styrene) blocks in a styrene-diene block copolymer. The poly (vinylcyclohexane) endblocks exhibit an increased Tg that occurs at 145xc2x0 C., which is a 50xc2x0 C. increase in the Tg of conventional poly (styrene) endblocks. Unfortunately, this process requires hydrogenation at elevated temperatures, which adds cost to the manufacturing process. Additionally, high temperature hydrogenation can lead to hydrogenolysis of the Cxe2x80x94C bonds of the polymer backbone; such a chain degradation mechanism affords polymers having reduced mechanical performance, poorer strength, despite the high Tg of the polyvinylcyclohexane blocks. There are other disadvantages with that approach; following exhaustive hydrogenation, the block copolymer is a completely aliphatic, amorphous material and as such has poor oil resistance. The driving force for phase separation in these polymers is reduced as polyvinylcyclohexane is more compatible with an hydrogenated diene rubber than is polystyrene. Because block copolymers rely on phase separation as the mechanism for strength formation, hydrogenation of the polystyrene block reduces strength.
What is needed is a new approach to increase the Tg of such poly(styrene) block copolymers, that does not have such other detrimental process disadvantages and offers a product with a better balance of properties.
Accordingly, the present invention relates to a hydrogenated block copolymer having the general configuration
Axe2x80x94B,
Axe2x80x94Bxe2x80x94A,
Axe2x80x94Bxe2x80x94Axe2x80x2,
(Axe2x80x94B)nX, where n is an integer from 2 to about 30, and X is coupling agent residue, or
Axe2x80x94Bxe2x80x94C and wherein
a. prior to hydrogenation each A and Axe2x80x2 block is a random copolymer of an alkenyl arene monomer and a 1,3-cyclodiene monomer, each B block is a polymer block of at least one acyclic conjugated diene, and each C block is a polymer block of an alkenyl arene monomer;
b. subsequent to hydrogenation about 0-10% of the arene double bonds in the A, Axe2x80x2 and C blocks have been reduced, at least about 90% of the conjugated diene double bonds in the B blocks have been reduced, and at least about 90% the 1,3-cyclodiene double bonds in the A and Axe2x80x2 blocks have been reduced;
c. each A, Axe2x80x2 and C block having an average molecular weight between about 3,000 and about 60,000 and each B block having an average molecular weight between about 30,000 and about 300,000;
d. the weight ratio of alkenyl arene monomer to cyclodiene monomer in each A and Axe2x80x2 block is between 1:1 and 99:1;
e. the total amount of A, Axe2x80x2 plus C blocks in the hydrogenated block copolymer is above 10%, preferably about 15 percent weight to about 80 percent weight; and
f. the glass transition temperature for the hard phase of the block copolymer is in excess of 105xc2x0 C.
The polymers of the current invention have surprisingly improved creep properties at all temperatures, but particularly are improved at temperatures above 60xc2x0 C. when compared with a traditional block copolymer known in the art. This improvement in higher temperature performance is achieved without increasing the polarity of the A block. In order to achieve these higher temperature properties, it is important that the cyclodiene be copolymerized in a nearly random way with styrene. If the monomers form separate blocks, the Tg of the styrene block will not be increased, whereas a random copolymer will result in an increase of the polystyrene Tg in proportion to the fraction of added cyclodiene. The randomness of the copolymerization may be characterized in terms of the average number (N) of consecutive cyclodiene repeat units in the copolymer (a number that can be measured using a Proton Nuclear Magnetic Resonance (H-NMR) method). To avoid getting a copolymer having separate polystyrene and polycyclodiene Tg responses, it is preferred to have less than 10, on average, consecutive cyclodiene repeat units in the copolymer (N less than 10). It is more preferred to have N less than 5. Such a copolymer segment will have an even more random structure.
In still another aspect of the present invention the polymers of the current invention may be blended with polyolefins and, optionally, process oils to form compounds which have improved high temperature performance when compared to blends with traditional styrenic block copolymers. With traditional block copolymers, properties of these compounds, such as compression set at elevated temperature, for example 70xc2x0 C. and 100xc2x0 C., are improved to a limited extent by increasing the molecular weight of the polystyrene block. By using the polymers of the current invention the properties for an equivalent styrene block molecular weight will be improved. Alternatively, a lower styrene/cyclodiene block molecular weight could be chosen to give the same properties as the traditional block copolymer. Polymers of the current invention with the lower molecular weight A block would have improved processability compared to the traditional styrene end block. Because of their improved temperature performance the polymers of the current invention have application in molded or extruded goods which experience high temperature environments such as automotive applications and which require high temperature sterilization such as medical and personal hygiene products. The resulting soft compound may also be used in over-molding applications onto a wide range of thermoplastic polymer substrates. Improved creep at body temperature will also make these polymers advantaged for personal hygiene applications such as elastic articles in diapers and clothing.
These applications may employ either the block copolymer by itself, or preferably, blends of the block copolymer with polyolefins, polyphenylene oxides, styrenic resins and optionally process oils. Suitable polyolefins would be chosen from the group of polypropylene homopolymers, polypropylene copolymers, polyethylene homopolymers, and polyethylene copolymers with olefins and with vinyl acetate or acrylic monomers. Styrenic resins would include polystyrene, ABS, HIPS, and high styrene random and block copolymers.